Filters that include a microstrip line using a superconducting film are low-loss filters and are expected to be applied to GHz-band high-power transmission apparatuses such as base stations for mobile communication.
However, the superconductivity of a superconducting film tends to deteriorate when power applied to the superconducting film is high; thus it has been difficult to apply such a superconducting film in high-power applications.
For this problem, a filter that uses a disk-shaped electrode pattern and prevents power to be applied to the filter from being high has been proposed.
Moreover, in order to obtain a steep filter characteristic, a technology has been proposed in which a multiple-stage filter is configured by arranging a plurality of resonators, each of which is provided with a disk-shaped electrode pattern, on a dielectric substrate and by coupling the resonators.
FIG. 1 presents a schematic structure of a superconducting tunable filter 10 disclosed in Japanese Laid-open Patent Publication No. 2008-28835.
Referring to FIG. 1, the superconducting tunable filter 10 is formed on a dielectric substrate 11. The superconducting tunable filter 10 includes a superconducting ground layer 12 that covers the back-side surface of the dielectric substrate 11, superconducting disk-shaped electrode patterns 13A, 13B, 13C, and 13D that are formed on the front-side surface of the dielectric substrate 11, a superconducting input-side feeder pattern 14A that is coupled to the superconducting disk-shaped electrode pattern 13A, a superconducting output-side feeder pattern 14E that is coupled to the superconducting disk-shaped electrode pattern 13D, a superconducting feeder pattern 14B that is used to couple the superconducting disk-shaped electrode pattern 13A to the superconducting disk-shaped electrode pattern 13B, a superconducting feeder pattern 14C that is used to couple the superconducting disk-shaped electrode pattern 13B to the superconducting disk-shaped electrode pattern 13C, and a superconducting feeder pattern 14D that is used to couple the superconducting disk-shaped electrode pattern 13C to the superconducting disk-shaped electrode pattern 13D. A dielectric plate 15 is provided apart from the front-side surface of the dielectric substrate 11 in such a manner that the dielectric plate 15 may be adjusted to be closer to or further away from the front-side surface of the dielectric substrate 11. The dielectric plate 15 enables adjustment of the center frequency of the superconducting tunable filter 10.
In the superconducting tunable filter 10 configured like this, the superconducting disk-shaped electrode patterns 13A to 13D prevent the intensity of an electric field from being high. Thus, the superconducting tunable filter 10 may be applied to high-power applications.
Moreover, holes 15A, 15B, 15C, 15D and 15E to that allow adjustment rods composed of a dielectric or magnetic material to pass therethrough are formed in the dielectric plate 15. Although not presented, adjustment rods composed of a magnetic or dielectric material are formed in such a manner that the adjustment rods may be adjusted to be closer to or further away from the superconducting disk-shaped electrode patterns 13A to 13D and the superconducting feeder patterns 14B and 14D through the holes 15A to 15E. With this structure, the bandwidth of the superconducting tunable filter 10 may be adjusted using the adjustment rods.
In the superconducting tunable filter 10 presented in FIG. 1, which is a related art superconducting tunable filter, the dielectric plate 15 is coupled not only to the superconducting disk-shaped electrode patterns 13A to 13D but also to the superconducting input-side and output-side feeder patterns 14A and 14E and the superconducting feeder patterns 14B to 14D. Thus, if the center frequency of the superconducting tunable filter 10 is adjusted by moving the dielectric plate 15 closer to or further away from the front-side surface of the dielectric substrate 11, coupling states of the superconducting input-side and output-side feeder patterns 14A and 14E and the superconducting feeder patterns 14B to 14D and the superconducting disk-shaped electrode patterns 13A to 13D also change. As a result, for the superconducting tunable filter 10 presented in FIG. 1, there is a problem in that adjustment of filter characteristics such as the center frequency and the bandwidth becomes complicated. Moreover, in the superconducting tunable filter 10 disclosed in FIG. 1, for example, the superconducting input-side and output-side feeder patterns 14A and 14E are coupled to curved peripheries of the superconducting disk-shaped electrode patterns 13A and 13D, respectively, from the outside. Thus, the area of a connecting portion, that is, the capacitance of the connecting portion is small, and it is difficult to ensure an appropriate connection. The same problem exists for the superconducting feeder patterns 14B to 14D. Thus, for the superconducting tunable filter 10 disclosed in FIG. 1, significantly suppressing loss is difficult.